Why is 8 shots from a Garand (or 10 shots from an SKS or other 8-10 shot semi auto rifle) is considered sub-par for a "go-to" rifle by most people, but 6-8 shots from a much slower and higher recoiling 12 gauge pump action shotgun is considered optimal?

Because, some people can't hit what they shoot at, the first 3 times they pull the trigger. Unless it is a scatter gun. There was a time when more than 1 shot without loading was all you had. I say use what you have and be proficient with it, or don't even think about using it for HD.

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You can't fix stupid....however ignorance can be cured through education!